A programmable controller (hereinafter referred to as “PLC”) used for factory automation (hereinafter referred to as “FA”) performs the control operation in such a manner that the on/off information is input from an input device such as a switch or a sensor, the logical operation is executed according to a sequence program (also called a user program) written in the ladder language or the like, and in accordance with the operation result thus determined, on/off information signals are output to an output device such as a relay, a valve or an actuator.
The PLC is connected to the input devices or the output devices either directly or through a network. In the case where a network system connected by a network is constructed, the on/off information is transmitted and received through the network. In the process, the information are transmitted by a master slave method in which the PLC normally acts as a master unit, and the devices as slave units.
In recent years, on the other hand, a fail-safe (safety) system has been introduced also in the PLC operation. Specifically, not only the PLC and the devices but the network has a security function built therein. The security function is the one for confirming safety and producing an output. In the case where the network system enters a hazardous state as an emergency stop switch is depressed or a sensor such as a light curtain detects intrusion of a person (a part of a human body), the fail-safe system is so activated that the safety system turns to the safety side and halts the operation. In other words, the aforementioned safety functions cause the system to produce an output and operate a machine only when safety is stored. Unless safety cannot be confirmed, therefore, the machine stops.
In a network system having the safety functions (safety network system) described above, the maximum response time from the occurrence of a fault, a hazardous situation or other unsafe state to the execution of the safety operation (device halt, etc.) is required to be kept constant. Specifically, in the case where information are transmitted by the master-slave method, as well known, the slave units return a safety response to a master unit sequentially in response to the request of the master unit, as shown in FIG. 1(a). In the shown case, three slave units constitute a network system. The on/off information handled here are I/O information for safety control in the form of normal (safe) or fault (hazardous). The maximum response time guarantees the time consumed for each communication cycle.
On the other hand, demand is high for collecting complementary information (unsafety information) other than the safety information described above, such as the slave unit status, the turn-on time and the number of times operated, for monitoring the slave units and the devices connected to the slave units. By acquiring these unsafety information, the life of the devices, for example, can be determined, and the devices can be replaced before they actually develop a malfunction and the system halts.
The unsafety information may be sent, for example, as shown in FIG. 1(a) in which only the unsafety information are transmitted in the communication cycle 1 while only the safety information is transmitted in the next communication cycle 2. According to this method, however, the safety information cannot be sent during the communication cycle 1, and therefore the maximum response time is as long as twice the length of the communication cycle.
As another method, as shown in FIG. 1(b), the safety response for transmitting the safety information in response to a request of a master unit can be returned with the unsafety information added thereto. Also in this case, as compared with the case of FIG. 1(a) in which only the safety response is returned, each communication cycle consumes a longer time. In any of these methods, therefore, the demand for shortening the maximum response time cannot be satisfied.
The object of this invention is to provide a safety network system, a safety slave unit, a safety controller and a communication method, and an information collecting method and a monitoring method for the safety network system in which the response time of the original safety signal is not delayed even in the case where the information other than the safety signal are transmitted or received while the system is in operation.